Known methods for producing low profile, small diameter fiber-optic endoscopes involve attaching a Gradient Index (GRIN) lens to the distal end of a fiber-optic image bundle. A GRIN lens is especially suitable for such applications since it is substantially cylindrical, and, when attached to the distal end of the fibre-optic bundle, forms a linear extension of the fibre-optic bundle.
A GRIN lens is cylindrical and has a refractive index that varies according to the distance from the axis, as shown in FIG. 1. The lens has two opposing, substantially plane, faces perpendicular to the axis. As well as refracting light at these surfaces, a GRIN lens refracts light as a result of the radial gradient in the refractive index of the lens material. In contrast to a GRIN lens, a more-common convex or concave lens has a constant refractive index and one or more curved surfaces at which light is refracted.
When attached to a fibre-optic bundle, a GRIN lens forms a real image of an object placed in front of the lens on the distal face of the fibre-optic bundle to which the GRIN lens is attached. The fibre-optic bundle transmits this image to the proximal end of the fibre-optic bundle where the image can be viewed directly, using a suitable eye piece, or additional lenses can be used to focus the image on the pickup of a video camera.
Optical instruments comprising a GRIN lens and a fibre-optic bundle provide a focussed image of an object placed at a specified distance, or placed within a range of a specified distance, from the distal face of the lens. This specified distance is called the working distance of the instrument. Typically, most GRIN lenses for small-diameter imaging applications, i.e., for use with an imaging bundle in the range of 0.25 to 1 mm in diameter, are designed for making instruments with a working distance of between five and fifteen millimeters (0.2 to 0.6 inches). One millimeter GRIN lenses for use in instruments with a working distances of 25-50 mm (1"-2") are also available to special order.
In small diameter endoscopes used in laparoscopic surgery, and in optically-equivalent instruments used in other applications, it is often desirable to have working distances considerably greater than 50 mm (2 inches). Instruments operating at such hyper-extended working distances cannot be mass-produced simply by attaching a nominal 1/4-pitch GRIN lens to the end of a fibre-optic bundle. Tolerances in the refractive index, in the radial profile of the refractive index, and in the length of the lens mean that the pitch of the lens has a tolerance that causes large variations in the sharpness of the image. Consequently, mass-producing instruments in this way would provide an unacceptably small yield of instruments having a satisfactorily sharp image.
To produce instruments for hyper-extended working distances with a satisfactorily sharp image, a GRIN lens having a nominal pitch of greater than 1/4 is attached to the distal end of a fibre-optic bundle, and the lens is then individually polished to length until the desired image sharpness is reached. Working distances beyond several inches (several hundred millimeters) can be achieved by this method, in which the polishing process essentially produces a GRIN lens of exactly 1/4 pitch (infinity focus). Because the need to polish the lens each lens/fiber-optic assembly individually, the known assembly process is time consuming and expensive. Also, because polishing is carried out after the lens has been attached to the fibre-optic bundle, the whole lens/fibre optic assembly must be rejected if the polishing process damages the lens.
The polishing process in the presently-known assembly method can damage the GRIN lens of the lens/fibre-optic assembly. More significantly, the polishing process can polish the lens beyond the point at which the image is optimally focussed. This destroys the ability of the lens to form an image on the distal face of the fibre-optic bundle of an object at any distance. Thus, because it includes polishing the lens to achieve an optimally-focussed image, the presently-known assembly method involves the risk that the lens polishing process will over-polish the lens, resulting in a useless lens/fibre-optic assembly. Because of this risk, manufacturers tend to stop polishing the lens at a point just before that which gives an optimally-focussed image. This improves yields, but produces lens/fibre-optic assemblies in which the image is not quite optimally focussed. Moreover, the process is time consuming and labour intensive.